This invention relates to an electrode assembly for a magnetic flowmeter or the like, and in particular to such an electrode assembly in which the electrode can be positively sealed in place and in which the electrode can be readily removed and replaced from the exterior of the meter body and in which the end of the electrode exposed to the fluid is "self-cleaning."
Typically, a magnetic flowmeter includes a meter body adapted to be connected in a flow pipe, the flow through which is to be measured. Magnetic coils are mounted on the meter body and are excited so as to generate a suitable magnetic field across the body. Two meter electrode assemblies are mounted on opposite sides of the body for sensing the voltage induced in a conductive fluid flowing through the magnetic field within the meter body. The voltage is proportional to flow and is suitably amplified to produce a signal indicative of flow.
Each electrode assembly typically includes an electrode and means for mounting the electrode through the meter body into contact with a conductive fluid in the meter body. Because the meter body is typically made of an electrically conductive material, the electrode is electrically insulated from the meter body. Generally, a liner of non-conductive material is provided in the meter body and a sleeve of non-conductive material is provided around the electrode. The liner may be rigid or compliant, and may be made of a number of materials, such as polytetrafluorethylene, polyurethane, butyl rubber, or ceramics. In some designs, the liner does not line the entire flow passage through the meter body, but consists of coated areas in the region of the electrodes. Such a construction is shown in U.S. Pat. No. 3,194,068, to Mannherz et al, for example. Regardless of the type of liner, however, it is essential that the electrode be positively sealed relative to the meter body.
In some electrode assemblies, such as is shown in U.S. Pat. No. 3,194,068, the electrode has at its inner end a head or skirt part which bears against the liner replacement of the electrode require that the electrode be removed from the interior of the meter body. This necessitates that the meter body be disconnected from the flow pipe system.
In, other known electrode assemblies, complicated and expensive constructions were utilized to sealably hold the electrode in place on the meter body and to permit the installation of the electrode from the exterior of the meter body. In U.S. Pat. No. 3,171,990, for example, expansion of a sleeve by a conical electrode is relied upon to seal the electrode.
Sealing the electrode is more difficult with some liner materials than others. When the liner material is polytetrafluoroethylene, it has been found essential to provide a skirt on the inner end of the electrode to provide an adequate seal, and the electrodes have not been removable from outside the meter body.
In certain applications, for example, when measuring sewage flow, the exposed faces of the electrodes must be repeatedly cleaned. To clean the electrode while in place in the meter body, ultrasonic cleaning techniques have been used. In one known version of an electrode adapted to be ultrasonically cleaned, shown in U.S. Pat. No. 3,479,873, to Hermanns, an ultrasonic transducer engages the electrode outside of the meter body and excites the entire electrode so as to clean its inner end which is exposed to the fluid flow within the meter body. In another ultrasonically cleaned electrode arrangement shown in U.S. Pat. No. 3,771,361, to Reznick, the ultrasonic transducer is positioned in a blind bore in the electrode, and bears on a relatively thin wall behind the exposed electrode face. Although this latter approach has substantial advantages, it does not permit the electrode to be removed without disassembling the meter from its flow pipe system.
It has been known that by properly shaping the end of the electrode, the electrode will be self-cleaned by the flow of the fluid being measured flowing over the electrode. However, the shape of these prior "self-cleaning" electrodes was such that the electrode could only be removed from the inside of the meter body. This, of course, required that the meter body be removed or disconnected from the piping system.
One of the objects of this invention is to provide an electrode assembly for a magnetic flowmeter or the like in which the electrode may be readily changed from the exterior of the meter body (i.e., a field replaceable electrode).
Another object is to provide such an electrode assembly in which the electrode may be readily and positively sealed with respect to its holder.
Another object is to provide an electrode which may be effectively and efficiently cleaned in situ by ultrasonic cleaning apparatus.
Another object is to provide an electrode assembly which is "self-cleaned" by the flow of fluid therepast so as to inhibit the accumulation of dirt thereon and so as to lessen the frequency (i.e., the number of times per given period of time) that the ultrasonic cleaning apparatus must be operated to clean the electrode.
Another object is to provide self-cleaning electrode assembly which may be readily used with meter bodies having various electrical insulation liners or coatings on the inner surfaces thereof.
Another object is to provide an electrode assembly in which the electrode is positively electrically insulated with respect to the meter body.
Another object is to provide an electrode assembly which is of relatively simple and economical construction and yet is able to withstand relatively high operating pressures without leakage.
Other objects will be in part apparent and in part pointed out hereinafter.